1. Field of the Invention
The present invention relates to an electron gun, and more particularly, to an electron gun for a cathode ray tube (CRT) having reshaped electron beam apertures.
2. Description of the Related Art
In general, an electron gun includes a triode consisting of a cathode structure, a control electrode and a screen electrode, a focusing electrode opposed to the screen electrode to form a pre-focusing lens and a final accelerating electrode opposed to the focusing electrode to form a main focusing lens.
If power is applied to a CRT, an electron gun emits electron beams from the cathode structure. The emitted electron beams pass through electron beam apertures of multiple electrodes and are focused and accelerated. The accelerated electron beams are selectively deflected by a deflection yoke installed at the cone portion of a bulb, and excite a phosphor screen on the inner surface of a panel, thereby displaying a picture image.
In the above-described CRT, in order to prevent enlargement or distortion of the spot of an electron beam landing on the phosphor screen due to a nonuniform magnetic field of a deflection yoke, a dynamic focusing method using a quadrupole lens, in which the cross section of an electron beam emitted from an electron gun is distorted in the opposite direction of the deflection magnetic field and the focus voltage applied to the electron gun is varied when the electron beam is scanned at the center or periphery of the phosphor screen, has been employed.
FIG. 1 shows the first embodiment of parts of electrodes of an electron gun based on the dynamic focusing method, and FIG. 2 is a view in elevation and in section of FIG. 1.
Referring to FIGS. 1 and 2, the focusing electrode of the electron gun includes a static electrode 10 to which a static focusing voltage VF1 is applied, and a dynamic electrode 100 which faces the static electrode 10 and to which a dynamic voltage DF varying in synchronization with a deflection signal is applied.
The electrodes 10 and 100 include outer electrodes 12 and 120 having separate electron beam apertures 11 and 110, and auxiliary electrodes 14 and 140 inside the outer electrodes 12 and 120 and arranged in-line, respectively. The auxiliary electrodes 14 and 140 have three separate apertures 13b/13a/13c and 130b/130a/130c for R, G and B electron beams so that electrons emitted from cathode structure are focused and accelerated by an electronic lens formed between each of the-respective electrodes according to application of a voltage.
Here, the diameters of the G electron beam apertures 13a and 130a formed in the center, among the three separate apertures 13b/13a/13c and 130b/130a/130c, are equal. However, the diameters of the R and B electron beam apertures 13b/13c and 130b/130c arranged at opposite sides of the G electron beam apertures 13a and 130a are different.
In other words, whereas the R and B electron beam apertures 13b and 13c are equal to the G electron beam aperture in diameter in the static electrode 10, the diameter of the R or B electron beam aperture 130b or 130c is greater than that of the G electron beam aperture 130a in the dynamic electrode 100.
Accordingly, the central axes of the R electron apertures 13b and 130b are spaced apart by a distance D, and the central axes of the B electron beam apertures 13c and 130c are also spaced apart by the same distance, as shown in FIG. 2. As described above, asymmetry in electric fields of the electronic lens formed between each of various electrodes makes it easier to adjust convergence.
However, when a dynamic voltage is applied to the final focusing electrode, that is, the dynamic electrode 100, since the focusing force of the final focusing electrode changes, the focusing force for converging three electron beams onto a phosphor screen changes accordingly. Thus, the capability of correcting convergence at the screen corner is deteriorated, thereby lowering picture quality.
In order to manufacture an electron gun having the electrodes 10 and 100, electrodes are arranged on a zig rod for assembling the electron gun, and spacers for maintaining a gap between each of the respective electrodes are interposed and then assembled. The assembled electrodes are fusion-fixed within the neck portion of a bulb by pressing buried portions at edges of the electrodes when bead glass positioned at both sides of each electrode is semi-fused.
However, in the above-described electrodes 10 and 100, the axis between centers of R electron beam apertures 13b and 130b and the axis between centers of B electron beam apertures 13c and 130c are spaced a predetermined distance D apart from each other. Thus, when the electrodes 10 and 100 are inserted into a zig, the R and B electron beam apertures 130b and 130c having relatively larger diameters become eccentrically disposed from the zig rod, which makes it difficult to attain alignment, resulting in poor assembling efficiency.
Although the electrode structure disclosed in U.S. Pat. No. 4,701,678 can easily adjust convergence, it is very difficult to fabricate.
In detail, as shown in FIGS. 3 and 4, facing electrodes 30 and 300 according to another conventional example are substantially trapezoidal laterally. In the electrodes 30 and 300, R electron beam apertures 32 and 320 and B electron beam apertures 33 and 330 are tilted toward the edges of G electron beam apertures 31 and 310 at a predetermined angle.
In this case, a problem is encountered in controlling tolerance since the R electron beam apertures 32 and 320 and the B electron beam apertures 33 and 330 are tilted from the top surfaces of the electrodes 30 and 300.
Also, the electrode structure disclosed in U.S. Pat. No. 5,027,043 exhibits deteriorated focusing characteristic.
In still another conventional electrode structure shown in FIGS. 5 and 6, outer electrodes 50 and 500 are provided and separate small, R, G and B electron beam apertures 52 and 520 are formed on top surfaces of the outer electrodes 50 and 500.
Here, enlargement portions 530 protruding from the rims of the R and B electron beam apertures 520b and 520c toward a G electron beam aperture 520a, are formed in the static electrode 500.
In this case, electron beams converge toward the enlargement portions 530. Thus, in spite of relatively easy assembling work, electron beam spots are locally distorted, thereby degrading the quality of a picture. Accordingly, the above-described electrode structure is not suitable for a high resolution CRT to which high-current electron beams are applied.
To solve the above problems, it is an objective of the present invention to provide an improved electron gun for a cathode ray tube (CRT) which can easily adjust convergence by changing the shape of electron beam apertures of electrodes, and which can reduce a position error when being assembled.
Accordingly, to achieve the above objective, there is provided an electron gun for a cathode ray tube having a triode consisting of a cathode structure, a control electrode and a screen electrode, a plurality of focusing electrodes for forming a pre-focusing lens unit for pre-focusing and accelerating R, G and B electron beams emitted from the triode, and a final accelerating electrode facing the focusing electrodes, for forming a main lens unit, wherein among R, G and B electron apertures of one of the focusing electrodes facing each other to form a quadrupole lens unit, to which an AC voltage having a relatively low peak, or a static voltage, is applied, enlargement portions which are asymmetrical with respect to the central axes of the respective electron beam apertures are formed into the rim of each of the R and B electron beams, so that the R, G and B electron beams are converged into one point even when the electron beams deviate to the corner of a screen.
Also, first and second vertically elongated polygonal or non-circular enlargement portions, with central axes spaced a predetermined distance from the centers of the R and B electron beam apertures, are formed into the rims of the R and B electron beams on opposite sides of the rims in the lateral direction.
Also, a third vertically elongated polygonal or non-circular enlargement portion, with a central axis coinciding with the center of the G electron beam aperture, is formed into the rim of the G electron beam aperture on opposite sides of the rim.
Further, the first and second enlargement portions deviate from the centers of the R and B electron beam apertures toward the G electron beam aperture.
The distance between each of the centers of the R and B electron beam apertures and the center of the G electron beam aperture is different from the distance from each of the central axes of the first and second enlargement portions to the central axis of the third enlargement portion.
Also, the distance between each of the centers of the R and B electron beam apertures and the center of the G electron beam aperture is greater than the distance from each of the central axes of the first and second enlargement portions to the central axis of the third enlargement portion.
Further, the sum of each of diameters of the R and B electron beam apertures and lengths of the first and second enlargement portions is different from the sum of the diameter of the G electron beam aperture and the length of the third enlargement portion, in view of the vertical direction of the electrode system.
According to another aspect of the present invention, there is provided an electron gun for a cathode ray tube having a triode consisting of a cathode structure, a control electrode and a screen electrode, a plurality of focusing electrodes for forming a pre-focusing lens unit for pre-focusing and accelerating R, G and B electron beams emitted from the triode, and a final accelerating electrode facing the focusing electrodes, for forming a main lens unit, wherein among R, G and B electron apertures of one of the focusing electrodes facing each other to form a quadrupole lens unit, first and second vertically elongated enlargement portions are formed into the rim of the R electron beam aperture on opposite sides of the rim in the lateral direction, and third and fourth vertically elongated enlargement portions are formed into the rim of the B electron beam aperture on opposite sides of the rim in the lateral direction, the respective enlargement portion having predetermined lengths in the normal direction of the horizontal axis of the electron beam apertures, so that the R, G and B electron beams are converged into one point even when the R, G and B electron beams deviate to the corner of a screen
Also, fifth and sixth enlargement portions having the same width and length may be formed into the rim of the G electron beam aperture on opposite sides of the rim in the lateral direction.
The width of each of the first and second enlargement portions is preferably different from the width of each of the third and fourth enlargement portions.